There has hitherto been reported an example where, even when a ferromagnetic layer is not subjected to anti-ferromagnetic coupling in relation to Current-In-Plane (CIP)-Giant-Magnetoresistance (GMR) that is acquired by causing an electric current to flow through the surface of a multilayer film of a sandwich structure of [a ferromagnetic layer/a non-magnetic layer/a ferromagnetic layer], a great magnetoresistance effect appears. Specifically, an alternating bias magnetic field is applied to one of two ferromagnetic layers with a nonmagnetic layer being sandwiched therebetween, to thus fix magnetization (the layer is called a “magnetization fixed layer” or a “pin layer”). The other ferromagnetic layer is reversely magnetized (called a “magnetization free layer” or a “free layer”) by an external magnetic field (a signal magnetic field or the like). Thus, a relative angle between magnetizing directions of the two ferromagnetic layers arranged with a non-magnetic layer sandwiched therebetween is changed, whereby a great magnetic resistance effect is achieved. A multilayer of such a type is called a “spin valve”. See the following related-art document 1 for detail.
Related-art document 1: “Phys. Rev. B45, 806(1992), J. Appl. Phys. 69, 4774 (1981)”
Since the spin valve can saturate magnetization at low magnetic field strength, the spin valve is suitable for use as an MR head. Although an MR head has already been put into practice, under present circumstances the rate of change in magnetic resistance of the MR head remains a maximum of about 20%. An MR element exhibiting a higher rate of change in magnetic resistance (hereinafter referred to as an “MR ratio”) has been required.
A TMR (Tunneling MagnetoResistance) element utilizing a tunnel effect is mentioned as a candidate for such an MR element. However, such an effect is exhibited as a result of electrons tunneling through an insulation layer. Hence, the TMR element usually has high resistance. When the MR head has high resistance, there arises a problem of a magnetic head incorporated in a hard disk drive generating large noise. In order to reduce the resistance, the thickness of a barrier layer must be reduced. However, it is known that, when the barrier layer is made thin, a uniform MR head cannot be manufactured, so that the MR ratio is deteriorated by pin holes. In relation to the TMR element, difficulty is encountered in achieving compatibility between low resistance and a high MR ratio. In the TMR, an electric current is caused to flow in a direction perpendicular to the film plane, and hence recording density of the hard disk is increased. When the size of the TAR head is reduced, resistance is increased further, which makes the head difficult to use.
In contrast, a CPP (Current-Perpendicular-to-Plane)—GMR element is mentioned as a candidate, wherein a sense current is caused to flow in a direction perpendicular to the film plane of an element. In the GMR element, electrons conduct through metal to thus appear. Hence, the GMR element has an advantage of low resistance. However, in the case of the spin spine valve film, the resistance to vertical conduction of an electric current is small. Hence, it is considerably important to increase the resistance value of an area in the film that contributes to spin-dependent conduction, thereby increasing the amount of change in resistance.
In order to increase the amount of change in resistance; namely, to improve the magnetoresistance effect, there has been conceived a technique for inserting a resistance adjustment layer including an insulator into a film of the spin valve. See the following related-art document 2 for detail.
Related-art document 2: “J. Appl. Phys. 89, 6943 (2001), IEEE Trans. Magn. 38, 2277 (2002)”
The spin valve is formed from an area that subjects electrons to spin-dependent scattering (a magnetization fixed layer/a spacer layer/a magnetization free layer) and an area where the degree of spin-dependent scattering is low (a ground layer, an anti-ferromagnetic layer, a protective layer, and the like). Provided that the resistance of a former area is taken as Rsd and the resistance of the latter area is taken as Rsi, the magnetoresistance effect of the spin valve can be expressed as ΔRsd/(Rsi+Rsd). As a result of attention being paid to an phenomenon of enhancing the magnetoresistance effect of the magnetoresistance effect becoming greater as Rsd is greater than Rsi, a resistance adjustment layer including an insulator is inserted as mentioned previously.
However, the enhancement of magnetoresistance effect solely achieved solely by the current confinement effect is limited. In order to further enhance the magnetoresistance effect, it is necessary to increase spin-dependent scattering factors of the magnetization fixed layer and those of the magnetization free layer. To this end, studies on half metal become active. Here, an expression “half metal” is generally defined as a magnetic material having the status density of only either up-spin electrons or down-spin electrons when the status of electrons in the vicinity of a Fermi level is viewed. When idealistic half metal can be materialized, there can be realized two statuses; namely, a state where resistance is infinite and another state where resistance is low, when the magnetization status of the magnetization fixed layer and the magnetization status of the magnetization free layer are anti-parallel to each other and when those are parallel to each other. Therefore, an infinite rate of change in MR can be achieved. In reality, even though thus far an idealistic state cannot be materialized, when a difference between the state density of up-spin electrons and the state density of down-spin electrons is greater than that achieved in the related-art material, remarkable and order-of-magnitude increases in the MR ratio are expected. However, half metal encounters a great problem of hindering commercialization. Specific problems are as follows. (1) In the case of perovskite-based half metal, an improvement in crystallinity is indispensable. However, in the case of a polycrystalline film employed in a magnetic head, an improvement in crystallinity is essentially impossible. (2) In general, a temperature at which a half metal characteristic can be maintained is low, and half metal hardly appears at room temperature. (3) There is the possibility of a half metal characteristic disappearing at an interface between different materials constituting a spacer layer interposed between the magnetization fixed layer and the magnetization free layer. Among them, problem (3) is fatal. Even when perfect half metal can be manufactured at room temperature, properties of half metal cannot be effectively utilized when a TMR film or a CPP-GMR film is formed by stacking half metal during formation of the spacer layer.
Incidentally, from the viewpoint of the magnetoresistance effect element, perfect half metal is not required. The essential requirement is an improvement in a ratio of spin polarization in the electrons conducting through the sense current; specifically, a ratio of spin polarization of electrons at Fermi level contributing to conduction. Proposed technique is to pay attention to the spin polarization ratio and to insert a function layer for modulating a band structure into the magnetization fixed layer and the magnetization free layer.
According to this technique, the function layer is formed from a very thin oxidation layer and the like. This means is based on a suggestion that, when an ultra-thin oxidation layer is inserted into the magnetization fixed layer or the magnetization free layer formed from metal, a spin is polarized in the vicinity of the interface. When the oxidation layer becomes thick, the resistance of an element is increased, and adverse effects, such as noise, are imposed on the element as in the case of a related-art TMR element. Accordingly, the oxidation layer is formed to a very-thin layer of the order of one atom, to thus enable an attempt to reduce resistance.
However, in general, when the function layer is formed to a thickness of the order of one atom layer as shown in FIG. 11, the function layer assumes the shape of an island, or a plurality of pin holes are opened. Thus, difficulty is encountered forming a uniform function layer. When holes are opened in the function layer, an electric current induced by electrons having passed through the holes turns into a shunt current, thereby ending in a failure to attain great spin-dependent scattering. As a result, the spin filtering effect is diminished. Accordingly, the function layer must be very thin and uniform.